Filament for electron tubes



w. mm M 6, M 0 5 V./ T 2 mm ,6 T 6, A 2 D W Y B m m 1, 3 M W x m Tm E w in? F m m P D WILLIAMS FILAMENT FOR ELECTRON TUBES Jan. 13, 1953 Patented Jan. 13, 1953 UNITED STATES PATENT OFFICE FILAMENT FOR ELECTRON TUBES Paul D. Williams, Palo Alto, Calif., assignor to Eitel-P/IcCullough, Inc., San Bruno, Calif., a corporation of California Application February 27, 1950, Serial No. 146,507

6 Claims. 1

My invention relates to filamentary cathodes for electron tubes, and more particularly to improvements in thoriated tungsten type filaments.

Thoriated tungsten wire comprising tungsten containing thorium oxide (thoria) is widely used in making filaments for electron discharge devices such as radio transmitting tubes. The practice is to form such filaments of solid thoriated tungsten wire, shaped as hairpins or other desired shapes, which filaments are mounted on a suitable stem structure. In order to improve the electron emission properties of the filament the thoriated tungsten wire is carburized to convert the outer portions of the wire to tungsten carbide, the practice being to heat the mounted filament in an atmosphere containing carbon, such as a hydrocarbon, to effect carburization.

The most serious disadvantage of such filaments is that they are brittle and fragile, due principally to the fact that a goodly portion of the solid cross-section of each filament wire is carburized. These fragile filaments constitute one of the weakest elements in the tube and are responsible for much loss due to breakage, both in commercial and military applications, particularly when the tube is subjected to shock and vibration.

The broad object of my invention is to improve the strength of thoriated tungsten filaments.

Another object is to achieve the improved strength without impairing the electron emission properties of the filament.

A further object is to provide a filament making material which may be readily fabricated in the initial stage, and which is easily handled and processed during tube manufacture.

The invention possesse other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of my invention. It is to be understood that I do not limit myself to this disclosure of species of my invention as I may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

Figure 1 is an elevational view of a filament structure embodying my improved filaments.

Figures 2 and 3 are enlarged cross-sectional views of a filament before and after carburization; and

Figures 4 and 5 are side views of the same.

Figures 6 and 7 are cross-sectional views of modified type filament before and after carburization; and

Figures 8 and 9 are side views of the same.

In terms of broad inclusion, my improved filament for electron tubes comprises a bundle of twisted strands of thoriated tungsten wire. In its preferred form the bundle is made up of several twisted wire ropes, each rope comprising twisted strands of the thoriated tungsten wire. Preferably only certain portions of the stranded filament are carburized to form a carburized zone running helically of the filament. To facilitate carburization of the filament, a carbon bearing thread is preferably inter-twisted with the wire, which thread is preferably wound helically of the filament axis. The filament material thus formed is suitably mounted on a supporting stem, after which the filament is heated to disintegrate the thread and carburize the wire.

In greater detail, and referring first to Figure l of the drawing, my improved filament for electron tubes is illustrated as a filamentary cathode structure comprising a plurality of parallel filaments Ill. The structure shown has a glass stem H carrying a pair of metallic leads l2 and a central standard l3 sealed to a press M of the stem. Flexible extensions M3 on the leads serve to make connections with the base of the tube. A cupshaped metal shield I'I fastened to standard I3 and embracing the press 14 serves to protect the glass stem.

In the filament structure illustrated, the filaments 10 are maintained under tension. The lower ends of certain of the filaments are welded to a fixed arcuate support l8 on one of the leads l2, and the lower ends of the remaining filaments are welded to a second fixed arcuate support 18 on the other lead I2. All of the filaments are welded at the upper end to a circular support [9 slidable on the standard l3. By this arrangement the filament heating current passes up through half of the filaments and down through the other half. The filaments are tensioned by a coil spring 22 encircl ng standard I3 and interposed between a stop 23 and a compression sleeve 24 extending upwardly and bearing against a cup 26 under the slidable support l9.

The above described stem construction is merely for purposes of illustration, it being understood that other suitable filament supporting structures may be employed.

The individual. filaments it embodying my improvements each comprise a bundle of twisted strands of thoriated tungsten wire. In the form shown in Figures 2 to 5 the filament is made up of a uniform circular bundle of twisted strands 21. The strands 21 are of an ordinary commercial grade of thoriated tungsten wire, the size and number of strands depending upon the diameter or filament desired. The diameter of the individual strands is preferably quite small so that a relatively large number of strands are provided in the bundle, the multiplicity of small strands being partially responsible for the improved strength compared to the usual filament of solid cross-section. For example, I prefer to use 21 strands of .002 inch diameter wire to produce a filament having electron emission properties similar to a .010 inch diameter filament of solid cross-section. Filaments made up of strands of .001 inch diameter wire have been used, but wires of such small diameter are harder to handle in a twisting machine. The strands may be twisted together in any suitable type of twisting machine, a sufiicient number of twists being provided to hold the strands together.

As above mentioned, the stranded character of the filament is partially responsible for the improved strength and pliability of the filament. Another very important factor, in combination with stranded feature, is the distribution of carbon in the final carburized product. The stranded filament is only partially carburized, preferably to provide a carburized zone running helically of the filament, so that a quantity of uncarburized strands are preserved to maintain the strength of the filament. Such distribution of carbon is conveniently accomplished by inter-twisting with the strands 21 a carbon bearing thread 23. This may be an ordinary white ootton thread and may be impregnated with aquadag, if desired, to increase the carbon content. The thread is preferably incorporated so that it lies as a helix along the filament, adjacent the outer strands of wire in the bundle, as shown in Figures 2 and 4.

The wire so fabricated and including the thread may be produced in long lengths by the wire manufacturer and supplied on the spool to the tube industry, in the same manner that solid thoriated tungsten filament making wire is now supplied to the trade. At the tube plant the stranded filament material is cut to length and the filaments are mounted by welding or otherwise to the filament supporting in the usual manner. During this mounting stage the stranded filaments have an advantage because they are more flexible and therefore more easily handled than filaments of solid cross section.

After mounting, the improved filaments are carburized by the simple expedient of heating the filaments in an oxygen free atmosphere, such as in hydrogen or in a vacuum, to a temperature suiiicient to disintegrate the thread and carburize the wire. Heating the filaments to a temperature of about 2'100 C. is adequate for this purpose. "I

The thoriated tungsten stransds adjacent the thread are thus carbluized in a zone 29 running helica-lly of the filament as indicated in Figures 3 and 5.

In order to obtain maximum strength I prefer to use a form of stranded filament shown in Figures 6 to 9 wherein the bundle is made up of several twisted ropes 3|, each rope comprising twisted strands 32 of the thoriated tungsten wire. For example, I have had good results using three twisted ropes, each rope comp-rising seven twisted strands of .002 inch diameter wire, to produce a filament having electron emission properties similar to a .010 inch diameter filament of solid cross section. As seen in Figure 8, the strands 32 of a given rope preferably have tighter twists than the twists between the ropes 3!. This provides an interlocking action where the strands bind together between the ropes.

In this case the carburizing thread 33 is preferably intertwisted with the ropes 3! so that the thread follows the twist of the wire ropes as shown in Figures 6 and 8. After heating, a carburized zone 34 is provided running helically of the filament as indicated in Figures '7 and 9. It will be noted, because the individual strands have tighter twists than the ropes themselves, that the carburized zone runs crosswise of the separate wire strands. In other words, the carburize'd zone runs across the strands rather than following individual strands as in the case first described. Thus, a given strand is carburized in sections only, and, since the remaining sections of the strand are interlocked between the ropes, an additional measure of strength is obtained in the filament.

I have found that filaments made in accordance with my invention have good electron emission efiiciency and filament life characteristics. I have also made extensive shock tests to com pare the improved filaments with those commonly used in the art. These comparative tests were made with electron tubes having filament stem structures similar to that shown in Figure 1, the tubes being substantially the same except that in one case the filaments were of the carburized stranded type shown in Figures '7 and 9, and in the other case were ordinary carburized solid wire filaments. A standard impact machine for electron tubes was employed in making the shock tests. The results showed that the ordinary filaments broke at an average of about 100 G. shock acceleration, while the improved filaments broke at an average of over 400 (3.. constituting an improvement of better than 300% in shock resistance.

Instead of employing a thread to carburize the filament, my improved stranded type filament may be carburized in the ordinary manner by heating it in a hydrocarbon atmosphere. For example, the stem shown in Figure 1, with the stranded filaments mounted in place, may be positioned in a bell jar containing a hydrocarbon atmosphere, such as benzene'and the filaments heated sufficiently to absorc carbon from the atmosphere. In this case the outer strands of each filament are uniformly carburized, leaving the strands in the core portions of the filament uncarburized. This type of carburized stranded filament does not have the maximum strength, but is still superior to an ordinary filament of olid cross section carburized in the same manner. Compared to ordinary filaments breaking at a average of about 100 G. shock acceleration, ti s t mentioned stranded filaments broke at an average of about 200 G., showing an improvement of about 100% in shock resistance.

I claim:

l. A filament for electron tubes comprising a bundle of twisted strands of thoriated tungsten wire, portions of the stranded filament being carburized, said carburized portions being so disposed relative to the filament axis as to provide a carburized zone running helically of the filament.

2. A filament for electron tubes comprising a bundle of twisted wire ropes, each rope comprising twisted strands of thoriated tungsten wire, portions of the stranded ropes of said filament being carburized, said carburized portions being so disposed relative to the filament axis as to provide a carburized zone running helically of the filament.

3 A filament making materal for electron tubes 5 comprising a bundle of twisted strands of thoriated tungsten wire, and a carbon bearing thread intertwisted with said strands.

4. A filament making material for electron tubes comprising a bundle of twisted strands of thoriated tungsten wire, and a carbon bearing thread intertwisted with said strands, the thread running helically of the filament.

5. A filament making material for electron tubes comprising a bundle of twisted wire ropes, each rope comprising twisted strands of thoriated tungsten wire, and a carbon bearing thread intertwisted with said ropes.

6. A filament making material for electron tubes comprising a bundle of twisted wire ropes, each rope comprising twisted strands of thoriated tungsten wire, and a carbon bearing thread intertwisted with said ropes, the thread running helically of the filament.

PAUL D. WILLIAMS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,478,087 Wilson Dec. 18, 1923 1,663,547 Gero Mar. 27, 1928 2,297,454 Berger Sept. 29, 1942 2,350,270 Atlee May 30, 1944 2,363,023 Warnke Nov. 21, 1944 2,399,757 Murdock May 7, 1946 2,400,893 Thurber et a1 May 28, 1946 2,406,419 Washburn et a1. Aug. 27, 1946 2,410,060 Goodale Oct. 29, 1946 2,456,761 Williams Dec. 21, 1943 2,497,111 Williams Feb. 14, 1950 

1. A FILAMENT FOR ELECTRON TUBES COMPRISING A BUNDLE OF TWISTED STRANDS OF THORIATED TUNGSTEN WIRE, PORTIONS OF THE STRANDED FILAMENT BEING CARBURIZED, SAID CARBURIZED PORTIONS BEING SO DISPOSED RELATIVE TO THE FILAMENT AXIS AS TO PROVIDE 